1. Field of the Invention
The present invention relates to a photoelectric conversion device and an image sensor using the same, and more particularly to a photoelectric conversion device used in an image input system such as an image scanner, a facsimile, a copying machine, or a composite printer obtained by combining these devices.
2. Description of the Related Art
Photoelectric conversion devices have been used widely for image input systems as described above. The photoelectric conversion devices can be divided into two classes, CCD and CMOS, based on difference in procedure of transmitting an input signal from a pixel to an output terminal.
In general, CCDs have been widely applied to image input systems compared with CMOS devices because of high SN ratio. A CCD photoelectric conversion device, however, requires plural power sources and consumes large power. Further, manufacturing cost for a CCD photoelectric conversion device is high compared with a CMOS photoelectric conversion device since its manufacturing method is different from general LSI manufacturing method. On the other hand, a CMOS photoelectric conversion device has such advantages that it can operate on a single power source the same for a general LSI, it consumes small power, and low cost manufacturing can be done since the LSI manufacturing method can be applied to.
In recent years, also regarding a random noise level, which has been considered higher than that of a CCD's, a high SN ratio close to that of a CCD's is achieved by the use of the technique disclosed in, for example, JP 09-205588 A. Further, a close contact image sensor of real scale system, to which plural IC chips having plural photoelectric conversion devices are mounted, is generally used as a CMOS image sensor. A close contact image sensor has been widely used in recent years since the sensor enables reduction in size and thickness of the image input system.
The reduction in size and thickness of the image input system, however, causes a problem in that noise which is radiated from a device such as a scanner, a facsimile, a copying machine, or a composite printer obtained by combining those devices, particularly, low-frequency 50 Hz/60 Hz noise which is radiated from a power source line, enters into a pixel of a photoelectric conversion device. In addition particularly in the case of the close contact image sensor, since a photoelectric conversion device is located immediately below a document platen on which a document is placed, there is also a problem in that noise from a human body enters a pixel when a hand is placed on the document platen. When the above-described external radiated noise enters a pixel of the photoelectric conversion device, the external noise is added to the photo-generated carriers, which are generated by incident light, and, as a result, an image output is affected by the noise. Accordingly, this is a serious problem.